The Mind's Eye

I work with a few blind people, and am always amazed at the speed in which they read Braille with their fingertips. I'm even more amazed at the way they can identify the contents of tactile diagrams - particularly when they have been blind since birth. Tactile diagrams are images that use raised surfaces so that blind and visually impaired people can feel them. These diagrams can be maps, graphs, pictures of human body parts ... anything you can think of. I've spoken to people who have produced such diagrams (and, indeed, I've produced one or two myself), and I regularly hear comments from them such as: "I don't believe the client will get anything out of this drawing" - particularly when the diagram is quite complicated. Personally, I always thought that for a blind person to make any sense of a tactile diagram, he or she must have had some sight in the past; surely a person who has been blind since birth, and has never seen a face, would make no sense of a diagram of a face in profile. I decided to do a little research.

I found a recent article in "Scientific American" - "How the Blind Draw", by John M. Kennedy (vol. 16 Number 3, November 2006). In this article, a number of experiments are described.
In Stockholm in 1993, 9 adults were tested - 3 men and 6 women. 4 were congenitally blind, 3 had lost their sight after the age of 3, and 2 had minimal vision. Each subject was asked to examine 4 tactile diagrams of line drawings of facial profiles; each diagram had a distinctive feature: one had a large nose, one had a big smile, one had curly hair and one had a beard. The subjects were asked to describe the most prominent feature on each display (smile, hair, beard, nose). Five of them - including one man who had been blind since birth - correctly identified all 4, and only 1 of the subjects identified none. On average, the group identified 2.8 out of 4. In comparison, when 18 sighted people were blindfolded and given the same test, they scored only slightly better, matching an average of 3.1 out of 4.

In another experiment - this time in Japan in 1993 - a study was made where early-blind subjects were asked to recognise raised-line outline pictures of common objects such as fish or a bottle. An average of 60 percent of the pictures were recognised correctly (sighted blindfolded subjects did a lot better, but only because they were generally more familiar with pictures.

Clearly, the ability to understand tactile diagrams varies significantly among blind people, but it is nevertheless incredible that people who have been blind since birth are able to recognise images in tactile drawings. Even more amazing is that blind people are able to draw pictures! The Scientific American article gives cases such as that of Betty who lost all her sight at the age of 2, yet is able to draw pictures that are easily recognised by sighted people: "Relying on her imagination and sense of touch, Betty enjoyed tracing out the distinctive shape of an individual’s face on paper." Are sight and touch somehow linked in the brain?

In a paper published in the science journal Nature in 1996 )"Activation of the Primary Visual Cortex by Braille Reading in Blind Subjects", Nature, 380:526-528 by Sadato et al.), PET brain scans were done on Braille readers to find out what happened in the visual cortex (the visual cortex is the part of the brain that makes sense of information coming into our eyes). The subjects of this study included people with normal vision and people who were blind (either congenitally or from a infancy).
In the first experiment, the subjects were scanned under two scenarios. In one scenario, the subjects were simply required to sweep their fingers back and forth over a rough surface covered with dots. In the second scenario they were given tactile discrimination tasks such as deciding whether 2 grooves in the surface were the same or different. Using PET scanning, blood flow in the visual cortex was monitored during each task, and compared to a rest condition when the subjects were scanned while keeping their hands still. For the sighted subjects, there was a significant drop in blood flow in the visual cortex during the tactile discrimination tasks (they were blindfolded). In contrast, the blind subjects experienced an increase in blood flow, but only during the discrimination tasks and not when they aimlessly swept their fingers over the surface.
The second experiment involved reading Braille. The blind subjects read strings of eight Braille symbols and had to decide whether the strings formed a word or a non-word. Again, as in the first experiment, the blood flow increased in the visual cortex during the Braille reading.
It is still unclear how tactile information activates visual cortex neurons in blind people. Louis Braille invented the tactile reading system because he believed that vision loss was offset by heightened sensitivity in the fingertips. It makes sense to think that since the visual cortex is not used by the eyes, it is commandeered by our sense of touch ... it would be a shame if nature allowed this "real estate" to simply go unused. I like to think of it in a different way: seeing is not something that we do only with our eyes. Seeing is only possible when you know what you are going to see, and it is the visual cortex's job to create the expectations for what we are about to see. In blind people, the "expectation" part of seeing is operational but there is no data coming in from the eyes ... but input can come from other senses. It is possible for our mind to arrive at the same mental picture of an object via senses other than sight - for example, touch - and, indeed, "seeing" also involves touching, but sighted people are unaware of it because the input from he eyes is so overwhelming.
Each of us has a "mind's eye".‎